This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
When a UE is in WLAN/Wi-Fi coverage and supports VoWLAN (also called as VoWi-Fi), the UE will obtain an Internet Protocol (IP) address from a Packet Data Network (PDN) (i.e., PDN address) and can use this PDN address to make a VoWi-Fi call.
Currently, a UE is always located under both WLAN/Wi-Fi coverage and mobile communication network coverage. For example, the mobile communication network can be a 2G (2nd-Generation), 3G (3rd-Generation), LTE or LTE-A (LTE-Advantage) network.
With the current technology, in a scenario where a VoWi-Fi call is going on but the UE is moving out of the WLAN/Wi-Fi coverage and finally cannot discover WLAN/Wi-Fi signal but is still under the mobile communication network coverage (e.g., LTE network coverage), the UE will try to establish a voice call over the LTE network (i.e., a VoLTE call). However, because no handover from WLAN/Wi-Fi to LTE network is available, the VoWi-Fi call is finally disconnected.